To add red tone to the light emission of white LED to impart warmness to the emission color, a phosphor capable of red emission upon excitation with near-UV to blue light of LED is required. While many research works have been made thereon, WO 2007/100824 describes a complex fluoride of the formula A2MF6 which is doped with Mn wherein A is Na, K, Rb or the like, and M is Si, Ge, Ti or the like.
The preparation method described therein is by dissolving or dispersing raw materials in hydrofluoric acid and heating for evaporating to dryness. Since the solution containing a high concentration of hydrofluoric acid must be heated at high temperature, a substantial amount of hydrogen fluoride which is harmful to the human body volatilizes off. The method is thus quite difficult to implement in the industry. Besides the phosphor thus obtained is insufficient in purity, particle shape, and emission properties, leaving a room for improvement.
WO 2009/119486 discloses a method of preparing a phosphor by mixing a single metal such as silicon with a mixed solution of hydrofluoric acid and potassium permanganate. The method has a low reaction rate and hence, a low productivity, so it is less viable in the industry.
It is known from JP-A H05-306115 that particulate mica with a high purity and uniform size can be prepared using potassium hexafluorosilicate (K2SiF6) as the raw material. Potassium hexafluorosilicate is, in turn, prepared by adding potassium chloride to hexafluorosilicic acid aqueous solution, as described in Encyclopaedia Chimica, Kyoritsu Shuppan Co., Vol. 8, p278, 1964.
However, this method is unsuccessfully applied to phosphor preparation. When manganese is added to the reaction solution, an undesirable change probably attributable to a valence change of Mn ion occurs. The reaction product has no emissive properties and is not useful as phosphor.